The absolute length units are fixed and a length expressed in any of these will appear as exactly that size.
|in||inches (1in = 96px = 2.54cm)|
|px||pixels (1px = 1/96th of 1in)|
|pt||points (1pt = 1/72 of 1in)|
|pc||picas (1pc = 12 pt)|
Relative length units specify a length relative to another length property. Relative length units scales better between different rendering mediums.
|em||Relative to the font-size of the element (2em means 2 times the size of the current font)|
|ex||Relative to the x-height of the current font (rarely used)|
|ch||Relative to width of the "0" (zero)|
|rem||Relative to font-size of the root element|
|vw||Relative to 1% of the width of the viewport*|
|vh||Relative to 1% of the height of the viewport*|
|vmin||Relative to 1% of viewport's* smaller dimension|
|vmax||Relative to 1% of viewport's* larger dimension|
|%||Relative to the parent element|